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Hi Jack,

I'm very familiar with Ed Fong's Tri-band antenna and I'm pretty sure that
there is only one model which consists of a dual-band J-pole for 2m/440 and
a helical wound, center-fed dipole for 220 inside a PVC pipe. The J-pole
itself is very robust but some care needs to be taken when taking apart or
putting together the antenna in order not to bend the 220 dipole.

However Comet makes several different models of Tri-band antennas (and not
all of them for same 3 bands).
I own one each of the following 2m. 220, 440 Comet Tri-band antennas:
CX-333, SBB-224, SBB-224NMO.
The CX-333 is my base station antenna and installed on top of a 10ft
antenna mast (putting the top of the antenna 20ft above the roof). I use
the SBB-224 and SBB-224NMO for portable operation (the former with a mast
mount with radials and the latter with a magmount on vehicles).

Regardless of model, the Comet antennas are very different designs
(end-fed vertical dipole) from the Ed Fong antenna and therefore different
results are to be expected. That part of your question is therefore easily
answered with a very definite: Normal!

I'm getting a VSWR of less then 3:1 on all bands for all of those antennas
when measured under good conditions. This means that I'm doing the
measurement outdoors and that there is nothing in the proximity of the
antenna that would react and change the results (the antenna is on top of
the 18ft MFJ-1919EX tripod+mast or on a mag-mount in the center of the car
roof). The "somethings" that commonly effect the result of antenna
measurements are people, wires (including the feedline) and trees.

I'd consider a 5:1 result as something that warrants further investigation
to find out why it happened.

For my purposes a "close enough" result is sufficient and therefore don't
put too much effort into increasing precision in my VSWR measurements.
If you are concerned about accuracy, be sure to upgrade the firmware of
the NanoVNA so that it includes the enhancements made over time (such the
number of steps used during calibration and measurement). While the
software is very good in interpolating, the most accurate results are
obtained when using the same steps (number of steps and frequency range)
for calibration and measurement and to keep the frequency range as small as
the desired measurement allows.
If 2m/220/440 tri-band antennas are what you commonly test, consider
saving a wide-band calibration in C0 and then one band-specific calibration
for each band of interest saved to C1,C2 and C3. Don't forget to recall the
appropriate calibration before switching to measure a different band.

73,
Thomas

Here is the method we use to measure very low impedances in PDNs (power
distribution networks).

The figure and equation were extracted from the paper at the link below
(authored by Istvan Novak and published at the 2003 Designcon in Boston).

The equation is reasonably accurate when the measured impedance is << than
50 ohms (say < 5 ohms). The parasites of the measurement cancel out during
the vna calibration allowing accurate measurements at very low impedances.

BTW, check out some of the other papers on the site. Many are relevant to
VNA use.

-Ray
WB6TPU

On Feb 21, 2021, at 7:30 AM, Arie Kleingeld PA3A <pa3a@...> wrote:

?The Low-Z measurements.

The way to do that is using S21 measurement and short CH0 to CH1. Then

switch the low-Z impedance parallel to that. Ofcourse calbration is
necessary for Z=infinite and Z=0, just like the normal S21 calibration.

Enclosed are Excel graphs of S11 en S21 together.


For you information: 0,2 ohm = 5 times 1 ohm in parallel, 0,33 ohm is 3

resistors of 1 ohm in parallel.

Just to give you an idea of the simple measurements and performance of

the H3.2 on HF.

Of course the S21- file needs to be saved and imported into Excel and

then calculate The S21 values to value of R.

73,

Arie PA3A

On Feb 21, 2021, at 7:30 AM, Arie Kleingeld PA3A <pa3a@...> wrote:

?The Low-Z measurements.

The way to do that is using S21 measurement and short CH0 to CH1. Then switch the low-Z impedance parallel to that. Ofcourse calbration is necessary for Z=infinite and Z=0, just like the normal S21 calibration.

Enclosed are Excel graphs of S11 en S21 together.


For you information: 0,2 ohm = 5 times 1 ohm in parallel, 0,33 ohm is 3 resistors of 1 ohm in parallel.
Just to give you an idea of the simple measurements and performance of the H3.2 on HF.

Of course the S21- file needs to be saved and imported into Excel and then calculate The S21 values to value of R.


73,

Arie PA3A

Here is a quick check - put the 50 ohm load that you used for calibration
and run the same scan as you did for the tri-band antenna. I have had
several dual band and tri-band antennas over the years and have generally
found the dual band will have acceptable VSWR on both bands, but the
tri-bands will generally be ok on 2m/440 and high on 220 or ok on 2m/220
but high on 440. I suspect that for VHF/UHF tri-band antennas to work well
on all three bands is not an easy thing to do.

--
David S

On 02/21/2021 11:30 AM DiSlord <dislordlive@...> wrote:


For recall need only use
RECALL->RECALL 0
or
RECALL->RECALL 1
and e.t.c

if device hang on recall - possibly problem in bad data on it, try reset config and clear settings (use console and run 'clearconfig 1234').
Or you can save some valid data in all slots
After start save data on all slot:
CALIBRATE->SAVE->SAVE1
CALIBRATE->SAVE->SAVE2
CALIBRATE->SAVE->SAVE3
CALIBRATE->SAVE->SAVE4

On 02/21/2021 6:30 AM Arie Kleingeld PA3A < pa3a@... > wrote:


Hi Roger and all,

This one attracted my attention. Thanks for the nice graphs and foto's
Roger. Love your test-jig.
About 4months ago I wrote an article about measuring high impedances
with the nano. Also did one on low impedances.

For the Hi-Z story:
The nano did pretty well on 47 and 470 ohm resistors . 4k7 was worse and
10k was far off. All using *S11 measurement*. I enclosed de S11
measurement on a 10k resistor.

Using a *S21 measurement (series)* gave a very good value voor de high
impedance 10k and that result is also enclosed. It is an Excel graph
where Rs+jXs were derived from the S21 data, and then converted to Rp
and Cp (so series to parallel). Amazing result by the nanoVNA, which is
an old version H3.2 .

I did the same KIND OF thing for low-Z measurement with S21 with
resistor in /parallel /so to speak. I tried 1, 0.5 ,? 0.33 and 0.2 ohm.
Came up with 0.33 ohm accurate measurement. And I did not take any
special precautions. ). The 0.2 was OK on lower frequencies and a bit
off at the high side of HF. That might be my mistake. I do not carry a
lab here and only a junkbox full of old fashioned solderable components.
I'll post the Low-Z results in another message if someone would like to
see it. Not enclosing them now because we don't need any confusion here
:-)


73,

On 02/21/2021 6:30 AM Arie Kleingeld PA3A < pa3a@... > wrote:


Hi Roger and all,

This one attracted my attention. Thanks for the nice graphs and foto's
Roger. Love your test-jig.
About 4months ago I wrote an article about measuring high impedances
with the nano. Also did one on low impedances.

For the Hi-Z story:
The nano did pretty well on 47 and 470 ohm resistors . 4k7 was worse and
10k was far off. All using *S11 measurement*. I enclosed de S11
measurement on a 10k resistor.

Using a *S21 measurement (series)* gave a very good value voor de high
impedance 10k and that result is also enclosed. It is an Excel graph
where Rs+jXs were derived from the S21 data, and then converted to Rp
and Cp (so series to parallel). Amazing result by the nanoVNA, which is
an old version H3.2 .

I did the same KIND OF thing for low-Z measurement with S21 with
resistor in /parallel /so to speak. I tried 1, 0.5 ,? 0.33 and 0.2 ohm.
Came up with 0.33 ohm accurate measurement. And I did not take any
special precautions. ). The 0.2 was OK on lower frequencies and a bit
off at the high side of HF. That might be my mistake. I do not carry a
lab here and only a junkbox full of old fashioned solderable components.
I'll post the Low-Z results in another message if someone would like to
see it. Not enclosing them now because we don't need any confusion here
:-)


73,

This next post is about measuring inductors and self-resonance.